1. Field of the Invention
The present invention relates to a radiation image processing device and a radiation image processing method which perform image processing on a radiation image, and a program which causes a computer to execute the radiation image processing method.
2. Description of the Related Art
In the related art, when imaging a radiation image of a subject with radiation transmitted through the subject, if the thickness of the subject is particularly great, there is a problem in that radiation is scattered inside the subject, and contrast of the acquired radiation image is degraded due to scattered radiation. For this reason, at the time of imaging of a radiation image, a scattered radiation elimination grid (hereinafter, simply referred to as a grid) is disposed between the subject and a radiation detector such that the radiation detector which detects radiation to acquire a radiation image is not irradiated with scattered radiation, and imaging is performed. If imaging is performed using a grid, the radiation detector is hardly irradiated with radiation scattered by the subject, and thus, it is possible to improve contrast of the radiation image.
If imaging using a grid is performed, a thin stripe pattern (moire) corresponding to the grid is included in the radiation image along with a subject image, and thus, an image is hard to view. For this reason, processing for eliminating the stripe pattern due to the grid from the radiation image has been known.
The grid has a configuration in which interspace materials, such as lead not transmitting radiation and aluminum or fiber easily transmitting radiation, are alternately disposed in a fine grating density of, for example, about 4.0 pieces/mm, and thus, is comparatively heavy. In portable imaging which is performed in a patient's room, the grid needs to be disposed between a patient lying on a bed and the radiation detector. For this reason, the burden of work to dispose the grid and the burden of the patient at the time of imaging are great. In the case of a convergence type grid, concentration unevenness may occur in the radiation image due to oblique incidence of radiation. A fine stripe pattern (more) corresponding to the pitch of the grid is recorded in the radiation image along with a subject image, and the radiation image may be hard to view.
Accordingly, processing in which imaging of a radiation image is performed without using a grid, and the effect of image quality improvement with elimination of scattered radiation using a grid is provided to the radiation image through image processing has been suggested (see U.S. Pat. No. 8,064,676B and C Fivez et al, Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation, 1996 IEEE, pp 339-342). In the methods of U.S. Pat. No. 8,064,676B and C Fivez et al, Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation, 1996 IEEE, pp 339-342, a radiation image is frequency-resolved into a plurality of frequency components, scattered radiation elimination processing for controlling contrast or latitude is performed on a low frequency component regarded as a component of scattered radiation, and frequency components after the processing are synthesized to acquire a radiation image with the component of scattered radiation eliminated. In the method described in U.S. Pat. No. 8,064,676B described above, the scattered radiation elimination processing is performed by multiplying the low frequency component by a gain according to the layer of the low frequency component and the pixel value of the low frequency component. The gain is a value less than 1, and has a smaller value for a lower frequency band or a brighter pixel value. In the method described in C Fivez et al, Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation, 1996 IEEE, pp 339-342, a lower frequency band is significantly suppressed in a geometrical progression manner using a table for converting a low frequency component according to the pixel value thereof.
According to the methods of U.S. Pat. No. 8,064,676B and C Fivez et al, Multi-resolution contrast amplification in digital radiography with compensation for scattered radiation, 1996 IEEE, pp 339-342, since a grid is not required at the time of imaging, it is possible to reduce the burden of a patient at the time of imaging and to prevent degradation of image quality due to concentration unevenness and moire.